1. Field of the Invention
This invention relates to HIV envelope polypeptides and vaccines containing the polypeptides.
2. Description of the Related Art
Acquired immunodeficiency syndrome (AIDS) is caused by a retrovirus identified as the human immunodeficiency virus (HIV). There have been intense efforts to develop a vaccine that induces a protective immune response based on induction of antibodies or cellular responses. Recent efforts have used subunit vaccines where an HIV protein, rather than attenuated or killed virus, is used as the immunogen in the vaccine for safety reasons. Subunit vaccines generally include gp120, the portion of the HIV envelope protein which is on the surface of the virus.
The HIV envelope protein has been extensively described, and the amino acid and nucleic acid sequences encoding HIV envelope from a number of HIV strains are known (Myers, G. et al., 1992. Human Retroviruses and AIDS. A compilation and analysis of nucleic acid and amino acid sequences. Los Alamos National Laboratory, Los Alamos, N. Mex.). The HIV envelope protein is a glycoprotein of about 160 kd (gp160) which is anchored in the membrane bilayer at its carboxyl terminal region. The N-terminal segment, gp120, protrudes into the aqueous environment surrounding the virion and the C-terminal segment, gp41, spans the membrane. Via a host-cell mediated process, gp160 is cleaved to form gp120 and the integral membrane protein gp41. As there is no covalent attachment between gp120 and gp41, free gp120 is sometimes released from the surface of virions and infected cells.
The gp120 molecule consists of a polypeptide core of 60,000 daltons which is extensively modified by N-linked glycosylation to increase the apparent molecular weight of the molecule to 120,000 daltons. The amino acid sequence of gp120 contains five relatively conserved domains interspersed with five hypervariable domains. The positions of the 18 cysteine residues in the gp120 primary sequence, and the positions of 13 of the approximately 24 N-linked glycosylation sites in the gp120 sequence are common to all gp120 sequences. The hypervariable domains contain extensive amino acid substitutions, insertions and deletions. Sequence variations in these domains result in up to 30% overall sequence variability between gp120 molecules from the various viral isolates. Despite this variation, all gp120 sequences preserve the ability of the virus to bind to the viral receptor CD4 and to interact with gp41 to induce fusion of the viral and host cell membranes.
gp120 has been the object of intensive investigation as a vaccine candidate for subunit vaccines, as the viral protein which is most likely to be accessible to immune attack. At present, clinical trials using gp120 MN strain are underway. However, to date no human vaccine trial has been of sufficient size to confirm or refute vaccine efficacy.
The development of candidate HIV-1 vaccines is burdened by the lack of in vivo or in vitro models of HIV-1 infection that accurately approximate the conditions of natural infection in humans. Several candidate HIV-1 vaccines [Berman et al.; J. Virol. 7:4464-9 (1992); Haigwood et al.; J. Virol. 66:172-82 (1992); Salmon-Ceron et al.; AIDS Res. and Human Retroviruses 11:1479-86 (1995)] have been described that elicit broadly cross-reactive antibodies able to neutralize a variety of diverse HIV-1 isolates in vitro. However, the relevance of in vitro assays to protective immunity in vivo is uncertain. Although several vaccines have provided chimpanzees with protection from challenge by homologous and heterologous strains of HIV-1, protection has not always correlated with in vitro neutralization assays carried out in T cell lines, or in lectin- and cytokine-activated peripheral blood mononuclear cells (PBMCs) [Berman et al.; Nature 345:622-5 (1990); Bruck et al.; Vaccine 12(12):1141-8 (1994); El-Amad et al.; AIDS 9:1313-22 (1995); Girard et al.; J. Virol. 69:6239-48 (1995); and Fulz et al; Science 256:1687-1690 (1992)]. While successful protection of chimpanzees is encouraging and has historically proved to be a reliable indicator of vaccine efficacy, the conditions of infection in all experimental models of HIV-1 infection differ significantly from natural infection in humans.
Experimental HIV-1 infection in vivo and in vitro both suffer from the limitation that the in vitro amplification of HIV-1, which is required to prepare virus stocks for in vitro or in vivo infectivity experiments, imposes a genetic selection that results in a spectrum of virus quasi-species that differ from the spectrum of variants present in the clinical specimens used to establish the culture [Kusumi et al.; J. Virol. 66:875 (1992); Meyerhans et al.; Cell 58:901-10 (1989)]. Because of these uncertainties, and even greater uncertainties related to the amount of virus transmitted, the site and cell type involved in initial replication, and the kinetics of virus dissemination, the ability of currently available in vitro or in vivo assays to reliably predict vaccine efficacy is questionable.
One of the candidate HIV-1 vaccines that have entered human clinical trials is recombinant gp120 prepared in Chinese hamster ovary (CHO) cells from the MN strain of HIV-1 (MN-rgp120) (Berman et al.; J. Virol. 7:4464-9 (1992)). To date, approximately 499 adults have participated in Phase 1 and 2 immunogenicity and safety trials of this vaccine. The data collected thus far suggest that MN-rgp120 is safe, immunogenic, and elicits high titers of neutralizing antibodies in greater than 95% of individuals immunized according to a 0, 1, and 6 month immunization schedule [Belshe et al.; JAMA 272(6):475-80 (1994); McElrath; Seminars in Cancer Biol. 6:1-11 (1995)]. However, during the course of these trials, nine vaccinees who received MN-rgp120 have become infected with HIV-1 through high risk behavior. Small trials, such as these, in populations with low rates of infection and minimally sized placebo control groups do not have sufficient statistical power to confirm or refute vaccine efficacy.
However, effective vaccines based on gp120 or another HIV protein for protection against additional strains of HIV are still being sought to prevent the spread of this disease.